Integrated chassis control generally comprises the integration or coordination of the operation of one or more vehicle controls, including control components, subsystems or systems, to improve vehicle dynamic performance and responsiveness. Integrated chassis control is used to improve overall vehicle dynamic performance by providing stability and handling predictability to a vehicle to support a broad spectrum of anticipated vehicle operators having a wide range of operating styles over a broad range of vehicle operating environments, including a wide range of vehicle dynamic states comprising accelerating, braking and coasting, combined with various types of steering inputs and maneuvers, tire characteristics and loading conditions, vehicle loading conditions, vehicle settings and other variables associated with the vehicle dynamic condition, and also including a wide range of roadway types, conditions and other variables associated with the operating environment. Integrated chassis control comprises controlling the dynamic response of a vehicle with respect to one or more of the degrees of freedom associated with vehicle movement in the operating environments mentioned by integrating or coordinating the operation of the basic vehicle controls, including steering, braking, engine, transmission and suspension controls, as well as other more advance vehicle controls, such as vehicle stability enhancement (VSE) systems, yaw control systems, traction control systems, antilock braking systems (ABS), throttle control systems, variable assist steering systems, variable ratio steering systems, active front steering (AFS) systems, variable suspension systems, and variable ratio steering systems. These vehicle controls are frequently computer-controlled using microcontrollers and a various electronic and electromechanical sensors, transducers, actuators and other components. An integrated chassis control system typically comprises one or more electronic controllers or control modules to implement a method or control algorithm for integrating or coordinating the vehicle controls and thereby improve vehicle dynamic stability and performance.
Because steering is one of the principal elements of vehicle control, the overall operating stability and dynamic performance of the vehicle is particularly characterized by its steering performance. In a given operating environment, steering stability and performance of a vehicle is in large measure characterized by its understeer and oversteer behavior. For a vehicle exhibiting oversteer, in a steady state condition, the steering angle required to negotiate a path of fixed radius decreases with increasing forward speed. For a vehicle exhibiting understeer, in a steady state condition, the steering angle required to negotiate a path of fixed radius increases with increasing forward speed. The steady state steering or cornering condition refers to a dynamic state or condition of the vehicle wherein the vehicle dynamic response(s) or output within the permitted degrees of freedom, such as the yawing velocity, lateral velocity, forward velocity, and rolling velocity, to periodic or constant inputs of the vehicle controls, such as the steering system, does not change significantly as a function of time for a given time interval. Transient state steering or cornering conditions include all dynamic states or conditions other than steady state conditions, such as, for example, where the steering control inputs are not periodic or are changing, or where the vehicle response(s), such as the yawing velocity, lateral velocity, forward velocity, and rolling velocity are changing.
A steering coefficient, Kμ, or indicator of the understeer or oversteer behavior can be can be calculated under steady state conditions for a vehicle from the following general steer equation:
                              δ          f                =                                            L              ⁢                                                          ⁢                              ψ                .                                                    V              x                                +                                    K              μ                        ⁢                          a              y                                                          (        1        )            where δf, L, {dot over (ψ)}, Vx, ay are the steering angle, wheel base, yaw rate, speed and lateral acceleration of the vehicle, respectively. This equation is based on a two degree of freedom bicycle model of a front wheel steer vehicle. The understeer calculation in this case encompasses the understeer gradient arising from the nominal cornering stiffness of the tire and the effect of the load transfer.
The above equation is valid in the linear range of the tire behavior, when the lateral force generated by the tire is proportional to the tire slip angle, during steady state conditions. Under non-linear tire behavior or during transient state conditions, the general steer equation becomes indeterminate and an oversteer and understeer behavior indicator cannot be determined using this mathematical expression.
Therefore, it is very desirable to develop a method and apparatus which may be used to characterize the steering behavior of a vehicle under non-linear, transient conditions and which is adapted for use in an integrated chassis control system.